This invention relates to a demodulating circuit for a medium speed facsimile, and more particularly to a carrier wave reproducing circuit in a synchronized detection system.
For transmission in a medium speed facsimile system, an AM-PM-USB modulating system is recommended by CC/TT (international telecommunication association), and a synchronized detecting system has been utilized for the demodulation of the transmitted signal. Such a demodulating method is carried out by extracting phase information from a modulated input signal received by a facsimile receiver. A typical circuit of this kind is illustrated in FIG. 1. As shown therein, a modulated input signal Si is applied to a synchronized detection circuit 1 and an orthogonal deformation compensating filter 2. The output of the filter 2 is applied to a frequency multiplication circuit 3. The output of the circuit 3 having a multiplied frequency is then applied to an input of a phase comparator 4. The phase comparator 4 receives the output signal of a voltage controlled oscillator 6 at its other input, and the compared result is applied to a loop filter 5. The output of the loop filter 5 is used for controlling the oscillation frequency of the oscillator 6. The comparator 4, loop filter 5 and the oscillator 6 constitute a phase synchronizing (locked) loop which delivers a phase synchronized (locked) signal from the oscillator 6. This phase synchronized signal is applied to a frequency divider 7 to be divided into the original frequency, and is then applied to a phase shifting circuit 8 which delivers a reproduced carrier. The reproduced carrier is then applied to another input of the synchronized detection circuit 1 which demodulates the modulated input signal by the use of the reproduced carrier into a demodulated signal So. For this reason, the circuits 2, 3, . . . 8 are collectively referred to as a carrier wave reproducing circuit.
According to the AM-PM-USB modulation system, orthogonal components appear in the modulated wave, and a slight deviation in the orthogonal components tends to create phase jitter. In order to eliminate the phase jitter, the aforementioned circuits 2-6 usually have complicated constructions, and the phase shifting circuit 8 is required for adjusting the phase relation between the input modulated signal and the reproduced carrier wave. Furthermore, the band width of the phase synchronizing loop must be narrowed for reducing the phase jitter even though the narrowing of the band width tends to lengthen pull-in time.